My shell-to-python heuristic is similar, though I'll write longer shell scripts if I find I need to run a lot of subprocesses (it's just unwieldy in python) and I'll write shorter python scripts if I have to do logic best expressed with objects, tuples, hashtables etc. (Technically bash has everything you need, but I would prefer not to).

Of course, there are languages like Ruby and Perl that would cover both bases pretty well, but I'm not willing to introduce a third scripting language to most teams and projects I work on. Not to mention that those languages have their own issues.